To improve fuel efficiency of a Hybrid Electronic Vehicle (HEV), a high performance magnet, which may produce higher output in a traction motor of the limited size, has been needed. In the conventional permanent magnet for a traction motor, a NdFeB sintered magnet as of a rare-earth permanent magnet has been used, but it includes expensive rare-earth elements such as Dy and Tb for higher thermal properties. Although, these elements provide higher thermal properties, they reduce magnetic force and are expensive. Accordingly, such conventional permanent magnets are not proper for the use in HEV. Therefore, it has been desired to develop a permanent magnet having higher performance with lower cost than the conventional rare-earth permanent magnet, by reducing the cost of magnet and by reducing the amount of expensive Dy element used therein and through enhancing magnetic force.
In conventional methods, for diffusing Dy or terbium (Tb), a pressed body is sintered and processed to near net shape, and subsequently a heavy rare-earth alloy or compound is coated thereon and heated for diffusion. Therefore, the process is complicated to continue. To the contrast, in the present invention, the process is reduced and more efficient than conventional process because sintering and heating processes are conducted simultaneously.
Previously, as a grain boundary diffusion technique, diffusion during sintering process has been attempted. In such a technique, grain boundary materials are coated on a pressed body, and the body is placed into a sintering furnace for a sintering process. During the sintering process, temperature is increased to 1000° C. or greater, and the vacuum atmosphere is generally of about 10−3 Pa or less. Since Dy evaporates at about 1000° C. and around 10−1 Pa, the amount of Dy wasted by evaporation is greater than the amount diffused on the magnet due to rapid evaporation in such condition.
Moreover, since Tb evaporates at about 1000° C. and around 10−4 Pa section, it does not evaporate during the sintering process. However, the diffusion efficiency of Tb is reduced since diffusion in the grain is generated due to substantially high temperature rather than diffusion to the grain boundary. Further, coating of heavy rare-earth on the pressed body may cause oxidation of the pressed body, and therefore, properties of the magnet may be deteriorated. Further, the conventional magnets were heated at argon (Ar) atmosphere after sintering, and therefore, the grain boundary diffusion materials may not evaporate or become vapor-deposited during the heating process.
The description provided above as a related art of the present invention is merely for helping in understanding the background of the present invention and should not be construed as being included in the related art known by those skilled in the art.